Transcript Linked___Genes

Linked Genes
Learning Objective
DOT Point: predict the difference in inheritance
patterns if two genes are linked
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Linked Genes vs.
Unlinked Genes
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Application of Mendel’s Rules assumes:
1. One allele completely dominates the other
2. All genes have 2 allelic forms
3. All traits are monogenic (affected by only
one locus)
4. All chromosomes occur in homologous pairs
5. All genes assort independently
Mendel's Law of Independent
Assortment
S Allele pairs separate independently during the
formation of gametes. This means that traits
are transmitted to offspring independently of
one another.
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Dihybrid cross - phenotypes
Independent assortment
An Interpretation from the Dihybrid cross
S During gamete formation, segregating pairs of
traits will sort independently.
S In other words, segregation of 2 alleles at one
genetic locus has no effect on the segregation
of 2 alleles at another locus.
S For example, the assortment of yellow and
green alleles has no effect on the assortment
of round and wrinkled alleles, and vice versa.
Discovery of Linkage
S William Bateson and R.C. Punnett were
working with several traits in
snapdragons, notably a gene for purple
(P) vs. red (p) flowers, and a gene for long
pollen grains (L) vs. round pollen grains
(l).
Bateson and Punnett’s snapdragons:
Flower Colour:
Pollen seed shape:
P = purple
L = long
p = red
l = round
True Breeding lines:
LLPP
llpp
x
PpLl
Phenotype
Number
Purple long
Purple round
Red long
Red round
284
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21
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Exp Ratio
F1
Exp Number
9
3
3
1
Crosses produced a deviation from the predicted
Mendelian independent assortment ratios.
What is going on????
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Linkage
S The parental phenotypes reappeared more
frequently than expected
S Bateson and Punnett hypothesized that there
was a coupling, or linkage, between the
parental alleles for flower colour and pollen
grain shape
S This coupling resulted in the observed deviation
from independent assortment.
Linked genes
S Genes are said to be “linked”
when their loci are found on
the same chromosome.
S It means that those alleles are
usually inherited together…
but not always!!
The offspring of this couple gets one of
each parental chromosome. The genes
are intact, so the alleles that are inherited
are of the “parental type”
Linkage
S Linkage is the tendency for a group of genes,
on the same chromosome, to be inherited
together via crossing over
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Linkage II
S However, for linkage to occur,
the genes, located on the same
chromosome, must be close
enough to each other.
S If the genes are close enough
to each other, and have a high
probability of crossing over
together, then they are
considered to be linked genes.
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Linkage III
S If genes are linked
together, then Mendel’s
law of independent
assortment does not apply.
S Genes, in this case, are not
passed on independent of
another gene.
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Recombination of Genes
S Recombination of linked genes is only
possible when crossing over occurs during
meiosis.
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Recombination
S In meiosis, genetic material can move from
one chromosome to another?
S It results in offspring having chromosomes
that are not identical to parental
chromosomes.
Morgan and Crossing Over
S Morgan proposed that the chiasmata visible on
chromosomes were regions of crossing over.
S Occurs between non-sister chromatids.
S Crossing over occurs in prophase of meiosis 1, where
homologous chromosomes break at identical locations
and rejoin with their non-sister chromatids .
Mechanism of crossing-over
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Recombination
S You can see that the probability of genes that are close
together (a and b) being separated during crossing over
is less than that of genes that are further apart (a and c)
Recombination
S In the example below, the mother is homozygous at both
loci, while the father is heterozygous at both loci.
G
a
G
a
G
a
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The codes found on the parental
chromosomes: Ga, gA
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If crossing over occurs in maternal
meiosis, there is no change, as the
alleles are the same on each.
HOWEVER, if there is crossing over in
the paternal chromosomes, and the G/g
alleles swap places, there will be
RECOMBINANT offspring with
chromosomes with the following allelic
combinations: ga or GA
g
A
Determining Whether
Genes are Linked or
Unlinked
DOT Point: Analyse the outcome
of dihybrid crosses with
independently inherited and linked
traits
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Test Cross I
S The test cross allows scientists to determine whether
genes are linked or unlinked.
S In order to perform a test cross one parent must be
heterozygous while the other must be homozygous
recessive.
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Test Cross II
S If the majority of the offspring have a genotype
similar to one of the parents, then the genes are
linked.
S If the majority of the offspring have a recombinant
genotype, then the genes are unlinked.
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Test Cross Example
Parent #1
S BbVv
S Grey with normal wings
Parent #2
S bbvv
S Black with vestigial wings
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Test Cross
BV
bv
bv
BbVv bbvv
Expect 575
ed
Results
Actual 965
Results
575
944
Bv
Bbvv
575
206
bV
bbVv
575
185
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Parental Genotypes
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965 (42%) +944 (41%) = 1909
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1909/2300 = 83%
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Recombinant Genotypes
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206 (9%)+185 (8%) = 391
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391/2300 = 17%
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If independent assortment was to occur, the
percentages would be 25% a piece.
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Based on the data, the recombinants arose
because of crossing over
Cross breeding
DOT Point: Explain how crossbreeding experiments can identify the
relative position of linked genes
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Gene Mapping
S As pointed out by T. H. Morgan’s student, Alfred
Sturtevant, who produced the first Drosophila gene map
in 1913.
S Each gene is found at a fixed position on a particular
chromosome.
S The basis of gene mapping is that since crossing over
occurs at random locations, the closer two genes are to
each other, the less likely it is that a crossover will occur
between them.
Recombination Frequency
S Amount of recombination between two genes
reflects the distance between them
S The greater the distance, the greater the
recombination frequency
S Greater chance of crossover between genes
Genetic Maps
S Recombination frequencies used to determine
relative locations on a chromosome
Linkage map for genes a, b, and c:
Genetic Maps Example
S For example, the recombination rate between the genes
B and P is 30.7%
B
P
0.0
30.7
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Genetic Maps Example
S The recombination rate between the genes P and R is
3.0%, so R is closer to P than B.
B
R?
P
R?
0.0
27.7
30.7
33.7
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Genetic Maps Example
S The recombination rate between the genes B and R is
35.5%, so B is further from R than P.
B
P
R
0.0
30.7
33.7
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Genetic Maps
S 30.7% + 3.0% do not equal 35.5%, this is because
recombinations are random.
S Short distances are more accurate than long distances.
B
P
R
0.0
30.7
33.7
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Example:
Genes A, B and C are linked. In a cross breeding experiment,
ABC was crossed with abc and the percentages of
recombinations were observed.
AB x ab
Results: AB and ab 85%
Ab and aB 15% (recombined through crossing over)
BC x bc
Results: BC and bc 95%
Bc and bC 5% (recombinations)
AC x ac
Results: AC and ac 90%
Ac and aC 10% (recombinations)
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A and B are 15 units apart
A ________________15 _______________ B
B and C are 5 units apart
B ____5___C
A and C are 10 units apart
A ________10 _________C
The resulting map is:
A __________10__________C______5_____B
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S Use this diagram to answer Questions
S In which gene map is the probability of crossing-over
C
between A and D greatest?
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S Use this diagram to answer Questions
S In which gene map is the probability of crossing-over
D
between A and D the least?
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S Use this diagram to answer Questions
S In which map are genes C and D most closely linked?
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B
S Use this diagram to answer Questions
S In map D, which genes are least likely to cross over?
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C and B
Chromosome mapping
DOT Point: Discuss the role of
chromosome mapping in identifying
relationships between species
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Genetic Maps
S If the two species are very closely related, then
their chromosome maps will be very similar.
S Chimpanzees and humans have DNA
similarities of more than 98%.
S This suggests that humans and chimpanzees
had a common ancestor in quite recent times.
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Genetic Maps
S A chromosome map that has been produced by
percentages of recombinations of genes during
crossing over does not completely represent a
whole chromosome.
S Chromosome maps based on recombinations
can only be used to produce the relative
positions of genes, not the absolute ones.
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